The present invention relates to a stencil printer or similar printer.
Today, a stencil printer is extensively used to form images on sheets and has a thermal head having a number of heating elements arranged in the main scanning direction.
While a stencil is conveyed in the subscanning direction in contact with the thermal head, the heating elements selectively generate heat in accordance with image data so as to form perforations or dots in the stencil. Then, the perforated stencil or master is wrapped around a rotatable drum. A sheet is fed toward the drum such that it meets the master wrapped around the drum. When the sheet is pressed against the drum via the master, ink is transferred from the inside of the drum to the sheet via pores formed in the drum and the perforations of the master, forming an image on the sheet. The drum is made up of a hollow cylindrical body having a number of pores formed in its circumferential wall, and a laminate mesh screen covering the cylindrical body and formed of resin or metal. The stencil has a laminate structure comprising an about 1 .mu.m to 2 .mu.m thick thermoplastic resin film and a porous support formed of Japanese paper or a mixture of Japanese paper and synthetic fibers.
The problem with the stencil printer is that the master is apt to slacken when wrapped around the drum. When a press roller or similar pressing member presses the sheet against the drum via the master, it crushes the slack and thereby causes the master to crease. This lowers the quality of an image printed on a sheet. To free the master from creases, it has been customary to provide the master with elasticity high enough for the master to move on the outer periphery of the drum when pressed against the drum, overcoming the viscosity of the ink. This successfully straightens the slackened master and thereby prevents the master from being creased. This is why the stencil is made up of the thermoplastic resin film and porous support.
However, the stencil having such high elasticity brings about the following problem. Assume that the porous support includes masses where Japanese paper or synthetic resins are entangled, or that thick fibers traverse the perforations of the master. Then, the ink is prevented from exuding via the master at such masses or perforations. As a result, an image printed on a sheet has its solid portion locally lost, has its thin lines disconnected, or has its thick lines blurred. That is so-called fiber marks appear on the printed image and degrade the image.
To reduce the fiber marks, the stencil may be implemented only by the thermoplastic resin film or may have its porous support of Japanese paper or synthetic fibers thinned. However, a stencil consisting only of the thermoplastic resin film or having a thin porous support has low elasticity. When this kind of stencil is heated for perforation, the resin film contracts and causes the stencil t o crease. Further, assume that the stencil or master with low elasticity and wrapped around the drum has a slack. Then, when the sheet is pressed against the drum via the master, the master fails to move on the outer periphery of the drum due to the viscosity of the ink. As a result, the slack of the master is crushed, causing the master to crease. Moreover, in a stencil printer of the type causing conveyor rollers to convey a stencil toward a drum while perforating the stencil, the stencil is prevented from waving due to its own elasticity during conveyance. Therefore, the stencil of low elasticity cannot be prevented from waving, so that it wraps around the drum without being straightened. Again, such a stencil or master wrapped around the drum is apt to slacken and therefore crease.
As for the laminate stencil made up of the resin film and porous support, even when the sheet is pressed against the drum by the pressing member via the master, the master is capable of sliding on the drum, overcoming the viscosity of the ink. Therefore, such a master creases little.
Stencil printers using the stencil consisting only of the thermoplastic resin film, but capable of smoothing it when wrapping it around the drum, have been proposed in, e.g., Japanese Utility Model Publication Nos. 60-21243 and 61-30866 and Japanese Utility Model Laid-Open Publication No. 60-119565. The printers disclosed in these documents each includes a stencil mounting device for straightening the slackened or creased stencil to be fed to the drum. The stencil mounting device has a guide for guiding the stencil to the drum, and a rotatable stretching member contacting the guide via the stencil. While the stencil is in movement, the stretching member is rotated by the stencil while exerting tension on the stencil in the widthwise direction of the stencil. In this type of stencil printer, the stencil is paid out from the stencil mounting device and fed toward the drum due to the rotation of the drum. At this instant, in the stencil mounting device, the stencil is conveyed along the guide member while being subjected to the tension ascribable to the stretching member. Consequently, the stencil can wrap around the drum without any slack or crease.
However, the conventional stencil printer using the guide member and stretching member has some problems yet to be solved, as follows. While the drum is in rotation, the stencil is sequentially paid out via a nip between the guide member and the stretching member. At this instant, tension acts on the stencil between the above nip and the drum. As a result, if the stencil has low elasticity, then it extends in the direction of stencil transport and creases in the lengthwise direction.
Further, the nip between the guide member and the stretching member is remote from the drum. So long as the master has high elasticity, the trailing edge of the master pulled out from the nip can wrap around the drum without slackening because the master is prevented from waving due to its own elasticity. However, when the master has low elasticity, the trailing edge of the master pulled out from the nip waves and wraps around the drum without being straightened, resulting in a slack. The slack would be crushed and would cause the master to crease, as stated earlier.
Japanese Utility Model Publication No. 62-20297 proposes a stencil printer including an elastic member whose upstream edge with respect to the direction of master transport is sequentially inclined from its intermediate portion to opposite sides in the above direction. With this configuration, the elastic member smooths the stencil outward in the widthwise direction of the stencil. Further, Japanese Patent Laid-Open Publication No. 7-125399 discloses a stencil printer in which a roller presses the master against the drum over the entire width of the master.